Using nuclear fission to generate electricity in boiling water reactors is known. Fissionable fuel is loaded into sealed fuel rods, which are in turn assembled into square arrays called fuel rod assemblies. The fuel rod assemblies include a lower tie plate for supporting the fuel rods and permitting the entry of water, an upper tie plate for fastening to at least some of the fuel rods and permitting the exit of water and generated steam, and a channel surrounding the lower tie plate, upper tie plate and fuel rods therebetween. The channel has the function confining the flow path between the tie plates and around the fuel rods. It is also the member that sets apart the steam generating flow path from the interior of the fuel bundles from the surrounding core bypass region.
In a BWR, water flows upward through the reactor core. As the water travels upward, it is heated and a part of it is converted to steam. The steam is used to drive a turbine and then cooled, recondensed and recirculated through the core. At the top of the core, .apprxeq.15% of the coolant by weight has been converted to steam, but the steam's volume is 70 to 80% of the total coolant volume.
In addition to cooling the reactor, water also moderates the fission reaction. Typically, the fission reaction produces fast neutrons. The chain nuclear reaction can only be continued by slow or thermal neutrons. The fast neutrons are slowed or "thermalized" by passage through the water coolant.
When coolant boils within a boiling water nuclear reactor, the neutron moderating water becomes less dense or has a high "void fraction." This high void fraction causes the moderation of the neutrons to occur more slowly.
To assure that a sufficient amount of water is present in the boiling water reactors for slowing neutrons, such reactors circulate a portion of the cooling water into so-called "bypass regions" which surround the fuel assemblies. The water which flows through these regions is not heated sufficiently to be converted to steam and therefore continues to act as a dense moderator (in the liquid state) for the fission reaction.
The configuration and purpose of the bypass region can be easily understood.
The so-called "core bypass region" is defined outside of the fuel bundles in the interstitial volume between the fuel channels. This region is not for the generation of steam. Consequently, the core bypass region--unlike the volume interior of the fuel bundles--is filled with liquid moderator only; steam is not generated in this location.
When water is admitted to the lower tie plate through a nozzle and connected plenum at the bottom of each fuel bundle, some water is bypassed through small nozzles to the core bypass region. Water also flows to the core bypass region through many so-called "leakage" paths. Water flow through the core bypass region does not contact the heat generating fuel rods. Speed of water flow through the core bypass region is designed to be fast enough to avoid generation of steam and maintain the presence of liquid moderator.
It is important to note that the flow of water through the core bypass region is in parallel with the flow of the coolant within the fuel bundles.
The water which remains in and flows through the core bypass does help moderate the nuclear reaction. Unfortunately, the water in the core bypass region adds nothing to the steam generation capability of the reactor. In fact, although the presence of liquid water is needed in the bypass regions is needed for neutron moderation, the energy required to pump the water through the core bypass region is otherwise wasted. It does not contribute to the steam generation and hence the energy output of the reactor.